SYDNEY—An Australian steelmaker is planning an up to US$700 million investment in the U.S., in the latest example of companies expanding production or restarting idle mills to make up for imports being priced out of the market in the wake of President Donald Trump’s import tariffs.

BlueScope Steel Ltd. said it is considering adding at least 600,000 to 900,000 metric tons of new steelmaking capacity at its North Star business in the U.S., which has become a major engine of profits at a company that for several years was focused...